(1) Field of the Invention
The present invention relates to a nonlinear compensating circuit for a radio apparatus, and more particularly, to a nonlinear compensating circuit for a microwave-band radio apparatus of which a built-in circuit has a nonlinear input/output characteristic.
(2) Description of the Related Art
With recent noticeable expansion of portable telephones, there is a demand for development of small-sized, lightweight devices with low power consumption.
Generally, a power amplifier used in a microwaveband radio apparatus has a characteristic such that its output power is proportional to the input power (i.e., the amplification factor is constant) while the input power is small, but becomes unproportional to the input power when the input power is large, causing a reduction in the amplification factor. Such nonlinearity is observed also in the case of phase; namely, while the input power is small, the output signal shows no phase deviation, but when the input power is large, the phase of the output signal advances or lags relative to that of the input signal. A frequency converter used in a radio apparatus also has a similar characteristic.
A conventional means of eliminating the influence of such nonlinearity is to provide a pre-compensating circuit for a power amplifier or the like of a radio apparatus arranged in a base station. Specifically, a distortion is detected through a comparison between the input and output sides of a power amplifier etc., a distortion-compensating distortion corresponding to the detected value is generated, and the generated distortion is combined or synthesized with the input signal for the power amplifier etc.
For portable telephones, the use of a pre-compensating circuit as mentioned above is not favorable because of its bulk, weight, and cost. Therefore, a large-capacity power amplifier or the like is used instead of additionally providing a pre-compensating circuit so that the power amplifier etc. may be used within a linear operation range even when it is supplied with large power.
In the case of the power amplifier or the like of a radio apparatus arranged in a base station, however, a distortion detecting section, a compensatory distortion generating section, a synthesizing section, etc. must be provided as the pre-compensating circuit, with the result that the apparatus is complicated in structure and is expensive.
Portable telephones equipped with large-capacity power amplifiers etc. are free from inconveniences that may be caused in the case where a pre-compensating circuit is used. However, such a power amplifier etc. inevitably have a large size and consume more power, obstructing reduction in size and weight of the battery. This adverse influence is great since the battery accounts for a large percentage of the bulk and weight of a portable telephone. This problem associated with portable telephones can be resolved if a pre-compensating circuit which is moderate in respect of bulk, weight and cost is available.